Question on SVT Style Driver

[Chipotle]

I'm looking at the 12BH7A cathode follower driver and the related bias supply. It looks like the power tube input (cathode follower cathode) is connected back to the grid of the cathode follower by about 390k or so of resistance. Isn't this a positive feedback loop? Which could lead to instability? The Heathkit W-6M has over 1 meg resistance in this area. Would it be advantageous to put another node on B-?

Thanks!

[loudthud]

Which SVT are you referring to? There are several versions and all the ones I'm looking at have a 47K from the cathode of the 12BH7 to -150V and a 47K grid stopper to each 6550 or 6146 grid.

Positive (phase wise) feedback is sometimes used in cathode follower circuits to increase the input impedance. It doesn't cause instability because the gain is less than one.

[Chipotle]

Thanks for the reply. I was looking at the 6550 schematic. I'm actually asking for an amp I've been building for awhile. I've got around 290k of resistance in that spot so I was concerned about instability, since I've got less resistance. What you're saying about the cathode follower gain makes sense, so I'll probably leave well enough alone. Thanks again.